Two hundred and seventy-nine consecutive patients referred for transient loss of consciousness, compatible with syncope, underwent head-up tilt to 70 degrees during polygraphic (EEG, ECG, pneumographic) and blood pressure monitorings. Vasovagal syncopes occurred in 28 patients with the following EEG changes: progressive slowing until the appearance of middle or high amplitude delta waves generalized and synchronous in 9 patients; delta waves suddenly followed by transient flattening of EEG activity in 16 patients. In 2 patients EEG could not be interpreted because of muscle and/or movement artifacts. Fifteen out of 28 patients exhibited a marked cardioinhibition, expressed by long-lasting cardiac pauses; a relationship between duration of EEG flat and duration of asystole was not found.